The invention is directed to fiber optic magnetic field sensor systems, and more particularly to methods and apparatus for stabilizing fiber optic magnetic field sensor systems.
Two-arm Mach-Zehnder fiber optic interferometers may be used for the sensing of magnetic fields by bonding a magnetostrictive material onto one of the fiber interferometer arms. The interferometer arm so bonded to the magnetostrictive material is used as a sensor fiber and the other interferometer arm is used as a reference fiber. When exposed to a magnetic field, the magnetostrictive material will stretch the sensor fiber while the reference fiber remains unaffected. As a result, a magnetically induced differential path length change or phase shift is introduced at the output of the interferometer.
The performance of this type of fiber optic magnetic field sensor depends on the characteristics of the magnetostrictive properties of the magnetic material. The desired material characteristics that are of interest to the magnetic field sensor applications are:
(1) High magnetostrictive constant which relates to the high sensor sensitivity. PA1 (2) Perfect or high degree of quadratic response of magnetostriction with respect to magnetic field, which relates to the high degree of linearity and the high sensitivity of the fiber sensor response. PA1 (3) Large maximum magnetic field beyond which the magnetostrictive response deviates from quadratic to some unacceptable level. The useable linear dynamic range of the magnetic material will be limited by this maximum magnetic field. PA1 (4) Minimal magnetic hysteresis to avoid ambiguity in determining the actual magnetic field without knowing its magnetic history. PA1 (5) Low magnetic noise from the sensor material which would enhance the low magnetic field measurement capability. One source of magnetostrictive noise is the Barkhausen noise which occurs at some discrete magnetic field levels corresponding to abrupt magnetic structural changes. PA1 (6) Minimal magnetic instabilities in terms of overall sensitivity due to surface oxidation and magnetic structural changes arising from repeated magnetic cycling over a moderate magnetic field range.
It is difficult, if not impossible, for a material to have all the desired features of high sensitivity, large linear dynamic range, uniform frequency response and, at the same time be magnetically stable over a large dynamic range of applied magnetic fields. The magnetic materials currently being used in the fiber optic magnetic field sensor are metallic glasses which must be thermally annealed near their Curie temperature in the presence of a strong magnetic field (&gt;500 Oe). These materials are chosen because of their magnetic softness, implying potentially high sensitivity for sensor applications. To date, it has been found that these materials have a working range of linear response below two Orsteds of magnetic field. In addition, the magnetic field annealing process is crucial in obtaining a smooth linear response in the 0-2 Orsted magnetic field range. There is also an indication that an initally smooth linear response will lose its smoothness through repeated cycling over a moderately large magnetic field.